The invention relates to non-pneumatic, structurally supported tires and wheels. More particularly, the invention relates to a compliant wheel that supports a load with its structural components and has pneumatic tire-like performance capabilities to serve as a replacement for and improvement over pneumatic tires.
The pneumatic tire is the best known solution for compliance, comfort, mass, and rolling resistance; however, the pneumatic tire has disadvantages in complexity, the need for maintenance, and susceptibility to damage. A device that improves on pneumatic tire performance could, for example, provide more compliance, better control of stiffness, lower maintenance requirements, and resistance to damage.
Conventional solid tires, spring tires, and cushion tires, although lacking the need for maintenance and the susceptibility to damage of pneumatic tires, unfortunately lack its performance advantages. In particular, solid and cushion tires typically include a solid rim surrounded by a resilient material layer. These tires rely on compression of the ground-contacting portion of the resilient layer directly under the load for load support. These types of tires can be heavy and stiff and lack the shock absorbing capability of pneumatic tires.
Spring tires typically have a rigid wood, metal or plastic ring with springs or spring like elements connecting it to a hub. While the hub is thereby suspended by the springs, the inflexible ring has only a small contact area with the road, which offers essentially no compliance, and provides poor traction and steering control.
Accordingly, except in limited situations, known non-pneumatic tires have not found wide use as substitutes for pneumatic tires.
A non-pneumatic, compliant wheel having performance characteristics similar to those of pneumatic tires, while improving on its disadvantages, would overcome the various deficiencies in the art and would be a welcome improvement.
A compliant wheel in accordance with the invention includes a compliant band that supports the load on the wheel hub and a plurality of tensile supporting elements that transmit in tension the load forces between the band and the hub. The tensile supporting elements are substantially non-load bearing in compression and provide pneumatic tire-like suspension to the hub.
According to a preferred embodiment, the tensile supporting elements are web spokes, sheet-like elements formed of an elastomeric material to provide tensile supporting force and flexibility when in compression. Other tension transmitting elements could also be used, for example, cords or cables, and the description of web spokes is not intended to limit the invention.
According to one embodiment, a compliant wheel includes a compliant band, a plurality of web spokes extending transversely across and substantially radially inward from the compliant band toward a wheel axis, and means for interconnecting the web spokes to a hub.
According to another embodiment, the compliant wheel includes a tread or wear portion formed on or mounted to the radially outer surface of the compliant band.
For the purposes of the following description, the term “hub” refers to any device or structure for supporting the wheel and mounting it to a vehicle axle.
The compliant band is formed of a material capable of deforming under load, including bending of the band, to envelope obstacles and to conform to a contact surface, such as a road or floor. In particular, bending deformation of the band under load forms a contact patch with the contact surface, which provides pneumatic tire-like transmission of traction and steering forces. One aspect of the compliance of the wheel material is that the amount of bending of the band relates to the magnitude of the load on the wheel.
The compliant band may be formed of an elastomeric material, such as natural or synthetic rubber, polyurethane, foamed rubber and foamed polyurethane, segmented copolyesters and block co-polymers of nylon. Preferably, the material has an elastic modulus of about 9 MPa to about 60 MPa. The band may be unreinforced, or may include a reinforcing ply to increase the band's circumferential inextensibility.
The web spokes interconnect the hub and compliant band and act in tension to transmit load forces between the hub and the band. This provides, among other functions, support for the mass of a vehicle. Load support forces are generated by tension in the web spokes not connected to the ground-contacting portion of the band. The loaded hub can be said to hang from the upper portion of the compliant band, which defines an arch supporting the load.
Preferably, the web spokes have a high effective stiffness in tension and a low effective stiffness in compression. The low stiffness in compression allows the web spokes attached to the ground-contacting portion of the compliant band to accommodate deformation of the ground-contacting portion of the compliant band without transmitting significant vertical load. The web spokes are relatively thin compared to their length, and typically, will bend in compression. The lack of compressive load support by the web spokes in the contact region allows the band to more easily form the contact patch and to more easily bend to absorb obstacles. In addition, because there is no direct connection from the ground to the hub, i.e., road shock must travel around the compliant band and through the tensioned web spokes, the compliant wheel has improved comfort and shock absorption compared to pneumatic tires.
The web spokes also transmit the forces required for accelerating, stopping, and cornering. The arrangement and orientation of the web spokes can be selected to obtain the desired function. For example, in applications where relatively low circumferential stiffness is desired, the web spokes can be arranged radially and in parallel with the compliant wheel axis of rotation. To increase stiffness in the circumferential direction, web spokes perpendicular to the axis of rotation can be added, alternating with the axis-aligned web spokes. Another alternative is to arrange the web spokes oblique to the compliant wheel axis to provide stiffness in both the circumferential and axial directions. Another alternative is to orient the web spokes to be in an alternating oblique arrangement, that is, in a zig-zag pattern when viewed on the equatorial plane. Of course, other similar arrangements could be used to tailor the circumferential stiffness of the wheel.
To facilitate the bending of the web spokes of the ground contacting portion of the tread, the spokes can be curved. Alternatively, the web spokes can be shaped during molding to have a predisposition to bend in a particular direction. Another alternative is to provide a connection between the hub and web spokes or between the ring and web spokes that acts in tension but allows relative movement of the web spoke in compression.
According to a preferred embodiment of the invention, a compliant wheel comprises a hub, a compliant, load supporting band disposed radially outward and concentrically with the hub, and a plurality of web spokes extending between the hub and the compliant band, wherein the compliant band comprises a reinforcing membrane or ply embedded in the band. Preferably, the reinforcing ply comprises cords aligned in the circumferential direction embedded in an elastomeric layer. According to this embodiment, the reinforcing ply acts to constrain the circumferential length of the band under load forces for better application of tension to the web spokes, which increases the load carrying capability.
According to another aspect of this embodiment, the membrane or reinforcing ply has a longitudinal tensile modulus greater than the elastic modulus of the band.
According to yet another aspect of this embodiment, the membrane or reinforcing ply is positioned approximately at the neutral axis of the compliant band. More preferably, the reinforcing ply is positioned radially inward of the neutral axis.